Eleanor Kellon, VMD
I would strongly suggest you titer first. A horse with a titer of over 1:1600 should not be vaccinated because of risk of severe reaction. This titer is also considered protective so no vaccination is needed.
Because IR horses are at higher risk of vaccine reactions in general, I would only use the intranasal vaccine. That's not without potential issues either and can cause symptoms, even abscesses, but not the systemic issues the injectable vaccine can cause.
Eleanor in PA